Story highlights. (CNN) Before the end of her short life, little Bella Bond was allegedly punched by her mother's boyfriend because he thought she was possessed, a law enforcement source involved in the investigation told CNN on Saturday.

It's unclear whether the punches killed the toddler who became known as "Baby Doe" after her remains, wrapped in a trash bag, were found in June on a rocky shoreline in Boston Harbor. "The tragedy of her death is compounded by the fact that her short life ended not by illness or accident, but we believe by an act of violence in the very place where she should have felt safest, in her home," Suffolk County District Attorney Daniel Conley said.

Michael McCarthy, who is not Bella's biological father, and the girl's mother, Rachelle Bond, were arrested this week in connection with the death. "We allege that McCarthy caused Bella's death, that he did so intentionally, that he and Bond took specific steps to keep Bella's death a secret and to avoid prosecution," Conley said. The mystery of the girl with adorable brown eyes and cherubic face -- as rendered by forensic artist Christi Andrews and distributed by authorities -- captivated the nation for months.

But it wasn't until a person staying with Bella's mother provided the crucial tip that investigators were able to identify her, the law enforcement official said. The tipster asked Bond about the child's whereabouts, and the mother admitted after being confronted that Bella was dead, according to the source. "This child, whose very name means beauty, was murdered," Conley said. On Friday, McCarthy, 35, was charged with murder, Conley said. Bond, 40, was charged as an accessory to murder after the fact. Conley declined to comment on the motive or manner of death. Court records show that Bond's criminal history includes arrests for heroin possession and prostitution, CNN affiliate WCVB-TV reported.

She wasn't 3 yet. Bella Amoroso Bond was just shy of 3 years old. "For nearly three months, we have endeavored to speak for a true innocent who could not speak for herself," State Police Col. Richard McKeon said. "Now we will give her justice.

After their arrests, the couple pointed fingers at each other over her death, Massachusetts House Speaker Robert DeLeo said. "It appears as though it was a situation of a boyfriend who was involved and that apparently, I think, mother and boyfriend sort of blaming each other in terms of who harmed the child," he said. Bond and McCarthy are set to be arraigned Monday. Neither they nor their attorneys have commented on the case. "Baby Doe" has been identified as Bella Bond, seen here in a Facebook photo. Parental rights terminated. McCarthy is in custody at a Boston area hospital, where he is getting treated for a medical condition; Bond is detained at a separate location, according to the law enforcement source.

The arrests came Thursday after a search of a Boston home. The state Department of Children and Families said it provided services for Bella in 2012 and 2013 following neglect complaints. It did not provide specific details.

Between 2001 and 2006, child welfare authorities terminated Bond's parental rights over two other children, an agency official said. One is in custody of the maternal grandmother; an unrelated family adopted the other, the official said. Yessiomara Torres, Bond's next-door neighbor, said the girl's mother told her she had lost custody of two other children and worried about losing custody of Bella, CNN affiliate WHDH-TV reported. "She said she didn't want anybody taking her away from her, that she is trying her best to be the best parent that she can be," Torres told the station. Authorities found the tiny remains on June 25 wrapped in a black trash bag. She was wearing polka dot leggings and lying next to a zebra blanket east of Boston's Logan International Airport. Police released photos of the blanket and leggings.

"We're gonna have to mourn together as a family through this, and stick together, and get through this the best way that we possibly know how to," Bella's biological father, Joe Amoroso, told CNN affiliate WHDH. "Somewhere in my heart I'm gonna have to find forgiveness and that time will come," he said.